1. Field of the Invention
The present invention relates to a positioning device for receiving a signal of a positioning satellite to perform position measurement, a positioning method and a storage medium in which a program is stored.
2. Description of the Related Art
Some techniques for completing position measurement at short times by a GPS (Global Positioning System) positioning device have been hitherto proposed as disclosed in JP-A-2007-248345 and JP-A-H10-031061. Furthermore, a device for automatically correcting the time of a clock by using time information contained in a signal of a GPS satellite has been also proposed as disclosed in JP-A-H08-015463 and JP-A-H05-249221.
In general, the position measurement based on GPS is performed as follows. First, the positioning device acquires transmission electric waves of a plurality of GPS satellites, and calculates pseudo-distances to the plurality of GPS satellites on the basis of respective positioning codes contained in the transmission electric waves. Furthermore, the positioning device receives ephemeris information contained in the transmission electric waves, and calculates the positions of the plurality of GPS satellites from the ephemeris information. Then, the position of the positioning device itself is determined on the basis of these results.
The electric waves transmitted from the GPS satellite are faint, and they are subjected to spectrum spreading. Therefore, in order to acquire and demodulate these electric waves, it is necessary to perform reception processing while establishing synchronization between the transmission side and the reception side.
The GPS satellite has accurate time information because of an atomic clock; however, a time counting circuit mounted in equipment of the reception side is not so accurate. Therefore, when a large error occurs in the counted time of the time counting circuit, it takes a long time to acquire electric waves due to unconformity in timing between the transmission side and the reception side.
Therefore, the inventor has considered a construction of intermittently receiving a signal from a GPS satellite during even a period for which position measurement is not executed and correcting the time of the time counting circuit to the time of GPS, thereby suppressing the error of the counted time to a small level. By this construction, the error of the counted time is not increased, and the electric wave transmitted from the GPS satellite can be quickly acquired.
Furthermore, it is also required to avoid needless signal reception because the reception of a signal from a GPS satellite requires relatively large power.
An object of this invention is to provide a positioning device, a positioning method and a program-stored storage medium that can quickly acquire a transmission electric wave of a GPS satellite on the basis of accurately corrected counted time, and also reduce signal reception frequency for time revision, thereby reducing power consumption.